Enterococcus faecium (Efm) caused very few infections in the past but these have increased markedly in recent years, including endocarditis and other serious infections, some of which are medically untreatable (e.g., >80% of nosocomial Efm are vancomycin resistant (VRE), almost all of which are ampicillin resistant, the traditional drugs of choice). This led to Efm's inclusion in the group of bad bugs from which we seem to have no ESKAPE. The factors behind the shift of Efm from commensal to pathogenic are not well understood but antibiotic resistance, mobile genetic elements and/or genes conferring increased pathogenicity or fitness have been blamed, since such genes are more commonly found in the Efm clonal complexes that account for almost all outbreak/nosocomial (hospital-associated (HA)) isolates. Our recent genomic comparisons showed that the species Efm consists of 2 distinct clades whose core gene sequences differ by >4%. Almost all HA Efm belong to one (HA) clade whereas almost all community-associated (CA) isolates from healthy volunteers group in the other (CA) clade; such core gene differences are not observed in E. faecalis although it too has HA clonal complexes. We also found that pbp5 is a core gene that encodes, in HA-clade strains, PBP5-R (ampicillin resistance) and varies from PBP5-S (ampicillin MICs <4) of CA-clade isolates by ~5%. Our molecular clock estimates indicate that the 2 clades diverged long before (300,000 ~ e1 million years) the modern antibiotic era. Thus, the recent increase of nosocomial Efm is due to replacement of CA by HA strains, rather than, as had been suggested, recent evolution within community-predominant (CA) strains. However, it is not clear what explains this replacement, why HA strains are seldom seen in healthy volunteers nor if HA isolates are more pathogenic. Perhaps CA strains actually colonize better, explaining their vast predominance in community feces, and perhaps it is simply the antibiotic resistance of the HA clade that allows its isolates to overcome CA isolates, grow to high density and then cause infections in the healthcare setting. An inherent difference between these 2 clades is resistance to ampicillin; of note, HA-clade ampicillin-resistant strains (which are also highly cephalosporin resistant) preceded nosocomial VRE emergence worldwide. The hypothesis of this proposal is that the core genome, and perhaps entirely pbp5-R of the HA clade, has allowed this clade to replace CA strains in patients receiving ?-lactams. In Aim 1, we will investigate which colonize better: HA-clade Efm strains or CA-clade Efm strains, and then determine the impact of ?-lactam use on promoting colonization by HA-clade isolates (PBP5-R) over CA isolates (PBP5-S) in the mouse GI colonization model. In Aim 2, we will examine the specific role of pbp5 alleles in Efm colonization during ?-lactam use by swapping pbp5-S and -R between CA and HA strains. The results could open new avenues for preventing infection or, if HA strains colonize better than CA ones even without ?-lactam antibiotics, new avenues to pursue in the future to unravel the contributions of other core and/or acquired genes.